The applicability of long-range GPR for monitoring seawater intrusion and validating groundwater flow models near LILW disposal facility

Prior to the construction of radioactive waste disposal facilities, a groundwater flow model was proposed to guide site monitoring, and it is updated based on new data collected during construction. Disposal facilities located in crystalline bedrock in coastal regions are particularly susceptible to seawater intrusion, which can lead to the formation of fracture zones and increased permeability. Therefore, the fluid movement may deviate from the predictions of the initial groundwater flow model.Conventional site assessment methods, such as borehole-based groundwater sampling, provide high accuracy but are limited in delivering comprehensive spatial interpretations. To address these spatial limitations, a long-range ground-penetrating radar (GPR) system equipped with real-time sampling was applied. This advanced GPR system enables deeper penetration, facilitating the evaluation of seawater intrusion zones and associated hydrogeological characteristics. The GPR survey identified seawater intrusion zones that showed a strong correlation with the electrical conductivity data of groundwater samples. The GPR results indicated that the groundwater flow model had overestimated the extent of seawater intrusion, necessitating modifications to improve its accuracy. In conclusion, GPR has proven to be a valuable tool for accurately assessing seawater intrusion zones and validating groundwater flow models. Furthermore, the GPR survey highlights its suitability not only for seawater intrusion assessments but also for long-term site monitoring in disposal facility settings.  Acknowledgement This study was supported by the Nuclear Safety and Security Commission (No. 1805020-0421-CG100).